In recent years, motor vehicles using natural gas as a fuel have attracted attention as low-pollution vehicles. Such a motor vehicle is mounted with a pressure vessel generally called a CNG tank (compressed natural gas tank).
The conventional pressure vessels for motor vehicles are made of a metal such as steel or aluminum alloy, but a pressure vessel made of a metal is heavy and shortens the drivable distance of the motor vehicle per unit weight of the fuel supplied to the motor vehicle. In addition, since the calorific value of natural gas per unit weight is only about one half that of gasoline, the quantity of natural gas which must be stored in a motor vehicle must be about double as compared with gasoline, if the drivable distance without additional fuel supply is to be kept as long as that for gasoline. This also increases the total weight of the motor vehicle, to further shorten the drivable distance of the motor vehicle. So, as a means for elongating the drivable distance of a motor vehicle, studies have been made to reduce the weight of the pressure vessel.
Japanese Patent Publication No. 5-88665 discloses a pressure vessel, in which a plastic inner shell capable of serving as a gas barrier is covered with a pressure resistant outer shell made of an FRP (fiber reinforced plastic). Since the pressure vessel is substantially made of plastics materials, it is very light in weight as compared with a metallic pressure vessel, and if it is used as a pressure vessel installed on a motor vehicle to contain natural gas, it can be expected that the drivable distance of the motor vehicle can be extended. However, on the other hand, since an FRP is fragile as compared with a metal, an impact due to a collision or any other cause can cause the pressure vessel to be burst instantaneously, to injure human bodies by its broken pieces or to cause an explosive fire due to leak of natural gas. Furthermore, if the progression of destruction of a car body by a collision is examined, it can often be observed that the same region of the car body receives impacts repetitively with the progression of destruction. So, even if the FRP pressure vessel is not burst by the first impact, the second impact applied to the same region can easily burst the FRP pressure vessel even if the impact energy is relatively low, and the same situation as caused when bursting is caused by one impact is brought about. Thus, the pressure vessel, especially the fuel pressure vessel of a motor vehicle is required not to be burst by one impact, but to be able to sustain its internal pressure even if it receives impacts repetitively. The prevention of bursting and the sustenance of internal pressure can, of course, be achieved if a higher safety factor is adopted. However, a higher safety factor increases the weight, to lose the greatest merit of weight reduction effect provided by the adoption of FRP, and raises the production cost.
Furthermore, U.S. Pat. Nos. 5,253,778 and 4,925,044 disclose a pressure vessel, in which a metallic boss is bonded to the opening of a plastic inner shell by an adhesive, and a pressure vessel, in which the flange of a metallic boss is coupled with and covered by an inner shell of plastics material covered with a pressure resistant FRP outer shell.
These pressure vessels are very light in weight as compared with a metallic pressure vessel, and if used as pressure vessels in motor vehicles, it can be expected that the drivable distance can be extended. However, since the coupling between a metallic boss and an inner shell of plastics material is not achieved by mechanical clamping, any impact applied to the metallic boss or its vicinity can disturb the close contact achieved between the metallic boss and the plastic inner shell, causing the gas in the pressure vessel to leak.